1. Field of the Invention
This invention relates to a novel forming method for wood to obtain a unique ligneous material having high hardness, a beautiful grain and an excellent external appearance. More specifically, this invention relates to a compression forming method for shaping a wood by using hydrostatic force of a pressurizing liquid.
This invention can be applied to a manufacturing process to economically produce, for example, a shaped ligneous material with improved surface hardness, or a sawn lumber with a beautiful grain and improved physical properties, or a so-called "fancy log" with decorative and complicated external appearance, without using a mold. The application of this invention is, however, not necessarily limited to these, and this invention is useful in other ways of application in industry.
2. Description of the Prior Art
Forming method, so-called "densification" for soft wood such as cedar, to improve the physical properties as well as its shape is known in the art. In the densification, a primary wood is heated in order to bring it into softened state, then compressed by using a mold to form it into a desired shape to obtain a shaped ligneous material with high surface hardness. The word "primary wood" means a log, lumber, or any other form of wood used as raw material which is to be treated in the process of this invention.
For example, a forming method has been practiced commercially, in which a timber of cedar is heated by hot water or saturated steam to around 100.degree. C. to bring the wood into a plasticized state. It is then compressed with the solid surfaces of a mold to form a pillar having polygonal cross section. Next, the shape is stabilized by cooling or drying the compressed wood and the wood is kept in the mold for many hours.
Also, another forming method is known in the art, in which a primary wood is plasticized by means of steam at a high temperature and pressure. Next, the plasticized wood is loaded between a pair of mold plates and compressed while steam under high or atmospheric pressure is applied. Then its shape is stabilized by leaving it in the mold under an atmosphere of high temperature steam for period of hours.
In either case, a shaped ligneous material having a higher density and a higher hardness in comparison with a primary wood is obtained. It should be noted that compression of wood by mold surfaces, when carried out to excess, can cause damage to the wood tissues resulting in local deterioration of hardness. Typically, however, compression of wood has been conducted almost exclusively by using the solid surfaces of a mold.
Moreover, as described above, it is absolutely necessary for a compressed ligneous material to be held in a mold closed under mechanical force for many hours, in order to stabilize the shape of the compressed ligneous material.
Up to this time, a few methods have been practiced commercially with the above purposes in mind. For example, cooling water is supplied through an inner path of the mold to cool down the temperature of compressed wood below the softening point, for a period of time. Then, the shaped material is taken out of a mold. In another way, after compressing a softened wood, the mold holding the compressed wood is treated in steam at around 180.degree. C. for a period of time, then the shaped material is cooled and removed of a mold.
Without said treatment by high temperature steam, the stress generated by the forming is retained in a compressed wood and will cause relaxation of compressed state of the wood when the shaped ligneous material is heated to a temperature above its softening point. That means, the volume of a compressed wood is recovered up to nearly the original volume, (i.e. this is so-called volume relaxation), and also a rebound in its shape takes place until it approaches to original shape.
It is evident from the above discussion that a long residence time in the mold is required for wood in the conventional densification methods, which will inevitably push up the production cost of shaped articles since manufacturing cost of a mold is generally high.
For the purpose of reducing a residence time in a mold, a method was proposed in an earlier development. That is, the use of a jig with sufficient strength to withstand a volume relaxation (called a shaping jig hereinafter). The jig is usually installed inside the mold, and softened wood is compressed inside the jig. When compression is over, the jig parts are firmly mechanically connected together along their edge lines, and the jig holding the compressed wood inside is immediately removed from the mold.
This method may be useful for manufacturing smaller shaped articles. But it can not be practical for production of large material, such as lumber for building a frame house, since rather thick, heavy metal walls are need for the construction of the jig.
In practice, after all, there has been almost no method, except use of molds or jigs, useful for the compression of wood and for the fixation of the resulting compressed structure or state. At the same time, no method has been proposed so far to utilize hydrostatic pressure for direct compression of plasticized wood in a liquid.